Computational Study of Lithium Titanate as a Possible Cathode Material for Solid-State Lithium-Sulfur Batteries

Abstract

LiS cells are currently built with metallic lithium as anode, a liquid electrolyte, and a cathode composed of a mixture of sulfur, carbon, and binder. While this type of cell produces good capacity during the early cycles, unwanted reactions with the electrolyte degrade the cathode and anode, making the whole cell not competitive with Li-ion batteries. A viable solution to mitigate this problem is the replacement of the carbon, binder, and electrolyte with a ceramic matrix, with high electronic and ionic conductivity. Lithium titanate (Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf>) spinel may be a potential candidate for the fabrication of composite cathodes, due to its mechanical robustness and its high electronic and Li-ion conductivity. In this paper, we present an ab initio molecular dynamics study complemented with experimental investigations, offering a novel interpretation for the Li-ion mobility in Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> and Li<inf>7</inf>Ti<inf>5</inf>O<inf>12</inf> as well as for the chemical reactivity of these materials with molecular sulfur. Also, we present a model for the passivation of Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> and Li<inf>7</inf>Ti<inf>5</inf>O<inf>12</inf> surfaces by lithium carbonate, addressing both Li-ion mobility at the interface and sulfur reactivity. On the basis of our results, the deployment of Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> and Li<inf>7</inf>Ti<inf>5</inf>O<inf>12</inf> materials for sulfur-based battery technology is questioned mainly by the lower Li-ion conductivity of the carbonate-passivated surfaces and by the chemical reactivity of Li<inf>7</inf>Ti<inf>5</inf>O<inf>12</inf> with sulfur molecules, which would lead to self-discharge, with resulting loss of capacity and inferior battery performance. (Figure Presented).